Multivibrator



March 2, 1948. B. B. JAcBsi-:N Er A1.. 435398 MULTIVLIBRATOR Filed June2, -1942 s sheets-smet 2 INVENTORS BENT auLow JAcoasEN MAURICE MolsELEVY FIG. a A

A TTOR/VEV S March 2, 1948. B. B. JAco-BSEJ Er Al. 2,435,808

' MULTIVIBRATOR l Filed June 2, 1942 5 Sheets-Sheet 3 'TINE' PatentedMar. 2, 1948 MULTIVIBRATOR Bent Blow Jacobsen and Maurice Mose Lvy,

London W. C. 2, England, assignors to Standard Telephones and CablesLimited, London, England, a British company Application June 2, 1942,Serial No. 445,510 In Great Britain July 8, 1941 (Cl. Z50-36) 1 12Claims.

This invention relates to the improvement of multivibratorsy forproducing regularly repeated electrical impulses, and in particularconcerns the stabilisation-"of theduration' and/or'of the frequency ofrepetition of the impulses.

It is well known thatmultivibrators of the ordinary type whose operationdepends upon the periodical chargeand discharge of condensers throughresistances in circuits involvingl thermionic valves and like devices,are liable to be unstable in their operation and in particular thefrequencyofrepetition of the impulses is often variable and dependentupon external factors such as battery supplies. In order to stabilisethe operation, it has in the past been found necessary to supplysynchronisingV signals of constant frequency from an external sourcesuch as a thermionic valve oscillator. This arrangement is costly andinconvenient and moreover usually requires that the frequencyV variationof the multivibrator should'remain within certain relatively narrowlimits toenable the synchronising source to function without getting outof step.

Accordingly, theV present invention Vprovides arrangements of thethermionic multivibrator type in'which the impulses produced areaccurately stabilised as regards duration, or frequency of repetition orboth, by simple circuit means operated by the impulses, and notcontaining any source of electrical power.-

The synchronising signals are obtained by means of delay networks-insuch manner that delayed impulses are obtained from one part of thecircuit and introduced into another (or the same) part and causethe-production of the impulses by a sort of trigger action, so that theperiod of repetition (or the duration) o f the impulses will bedetermined by the time delay in the network, which can be veryaccurately fixed. The delay mayV either be produced by directtransmission through the network, or by reflection at the distant end,where the terminals may be short circuited, or left open or may beconnected to some impedance calculated to produc a desired type ofreflection.

It is 'also possible by suitable design or termination of the delaynetwork, to produce delayed impulses of modified form for particularapplications.

' stabilisation may also be-eifected in a diierent manner, whereby theoscillations produced in a resonant circuit tuned to the repetitionfrequency (or one of its harmonics) are used as synchronis- Work. Y

In one embodiment of the invention, delay networks are introduced intothe circuit of a well regards frequency of repetition, or duration, or

both.v Y v In another embodiment, a multivibrator com"- prises anamplifier in which two timing circuits have been suitably located, andva delay network' forstabilising the frequency of repetition and/or theduration of the impulses. The amplier may beof a. type involving acathode follower valve or valves, and adapted for producing impulses ofvery high peak power by an arrangement of high efficiency involving theuse of plate potential much above the normal values specified for thevalves.

In a further embodiment, a multivibrator adapted asv a harmonicgenerator comprises an amplifier containing in addition to the twotiming circuits, a resonant circuit used for providing the'synchronising signals.

',Ihey invention will be better understood after a consideration of thefollowing detailed descriptiton and withY reference to the accompanyingdrawings inwhich Figs. 1 and 2 show block schematics of multivibratorsinvolving delay networks;

l Fig. 3 shows an embodiment in which delay networks are applied to acommon known type of multivibrator circuit.

Fig. 4 shows operation curves of the circuits; and Figs, 5, 6, 7, 8 and9 show additional embodiments of our invention.

In the various circuits shown in the above' quoted figures certainaccessories necessary for the operation of the valves but not essentialto the present invention are indicated. Thus, positive plate batteriesand negative grid biassing batteries'for valves are indicated by theusual symbols, which' however are not to be taken as implying anyparticular method of supplying the necessary potentials nor anyparticular values of the same. Any arrangements appropriate to thevalves and circuits may be used and would be provided Suitably by thoseskilled in the art. The valves are shown as triodes with indirectlyheated oathodes (with the heating circuits omitted for clearness) butvalves with any number of electrodes may be used as may be convenientaccording to circumstances.

' According to the present invention the synchronizing signals which areapplied to the grids so 'that they are suddenly brought to the cut-offpoint at the right time are obtained from the multivibrator impulsesthemselves by means of delay networks (or other means to be explainedlater) instead of from an external source, and it becomes thereforepracticable to stabilise simultaneously the duration of the impulse andalso the repetition frequency which, according to the methods hithertoused, would in general require two external synchronising sources. shownin block schematic form two methods by which synchronisation is obtainedby means of a delay network. M is a multivibratorcircuitof any knownform and L is a delay network which may be in the form of an articialline characterizable as having linear bilateral impedances and ispreferably such that the delay producedis. substantially independent offrequency overv the important working range though this is not' essen-In Fig. 1 are 4 tail owing to the different arrangements for picking upand delivering the impulses. It will be noted however that the reflectedimpulses will be transmitted right back to the input circuit 2 of L2.This is not necessarily objectionable, and even may be useful, but ifdesired the effect can be largely avoided by terminating the outputcircuit 3 in a suitable manner to produce reflected "impulses of smallamplitude.

L2 lin Fig. 2m) and will function as described to control eithertheduration of the impulses or the repetition frequency. Likewise thenetwork L tial. In Fig. 1(a) the network is shown with its l inputcircuit 2 connected to some suitable point in the multivibrator fromwhich impulses can be picked up, and with its output circuit 3 connectedto some other point where the impulses delayed by transmission throughthe delay network can be delivered in order to unblock the circuit. InFig. 1(1)) the output circuit 3 is not connected to M but is shortcircuited (or left unconnected) so that the impulses picked up from Mare reflected back to the same point where the unblocking may bearranged to occur. In some cases it may be desirable to terminate theoutput 3 with some suitable impedance in order to modify the reiiectedimpulses. ,g

The manner in which these networks stabilise the operation of thecircuit will be understood from the explanation given below inconnection with Fig. 2.

With the arrangements indicated in Fig. 1, it will only be possible tostabilise either the duration of the impulses. or the frequencyofvrepevtition. If it should be desired to stabilise-both, arrangementsshown in Fig. 2 involving two delay networks (or the equivalent of twonetworks) may be used. In Fig. 2(a) two delay networks Ll and L2 areconnected with their input circuits 2 to suitable points in themultivibrator circuit M, their output circuits 3 being arranged for theproduction of suitable reflections. The delay network LI may, forexample, be used to stabilise the duration of the impulses, and itsoutput circuit 3 will then preferably be short circuited. The network L2will then be used for stabilising the frequency of repetition and itsoutput circuit will preferably be left open. Y

In Fig. 2(1)) is shown an alternative arrange-y ment whereby the twonetworks Li and L2 are themselves each a part of a single delay network,having an input circuit 2, an output circuit V3 and a tapping point t.The input circuitZ Yis connected to a suitablev point inthe-multivibrator M for picking up the impulses, and the output circuit`3 is suitably arranged for reecting impulses arrivingthere. The tappingpoint 4 -is connected to some other suitable point in the multivibrator.

The arrangement of Fig. 2(b) may thus be regarded asa combination ofFigs. lui)` and 1(1)). The network L2 in Fig. 2(1)) may control thefrequency of repetition by means of impulses dem layed by'transmissionfrom the input circuit 2 to the tapping point fl, and duration oftheimpulses will then be controlled by Ll by means of impulses reected backto fromV the output circuit 3. The operation will be in principle asalready explained above, but will differ in de- .in Fig. im) mightcorrespond to L2 in Fig. 2(b) and would-then control the repetitionfrequency.

By suitable arrangement of the connections it could Yclearly be made tocontrol instead the duration'f the impulses.

In Fig. 3 is shown a multivibrator circuit to which `delay networks havebeen added for controlling the operation according to Figs. 1(1)) or2m). Two such networks Ll and L2 are shown.

The operation of the network may be understood with reference to Figs.'3and 4. Inv Fig. 4, curves 4A and 4B represent the variation with time ofplate potential and grid potential for the valve Vl, and 4C and 4Drepresent the same curves for valve V2. All the curves relateto the sametime axis. The grid of valve Vl is supposed to-be biased somewhat abovethe cut-olf point, which is indicated by the dotted line in Fig. 4B. Thecut-oif point of V2 is also shown in Fig. 4D by the dotted line and itsgrid is suitably biased as explained below.

Referring to Fig. 4D, the grid potential of V2 is supposed to havereached the cut-off point at time t3 after the previous cycle. Justbefore this moment the plate potential will be at itsV maximum value +E1because the plate current is zero (Fig. 4C) The grid potential of Vlwill be substantially equal to the bias voltage -e, (Fig. 4B), andthe'plate potential will be at a relatively low value +`E2 because ofthe anode current which is flowing (Fig 4A) However, as'soon as thepotentialfofthe grid of V2 reaches the cut-off, the plate current beginsto ow andthe potentials of the grids and plates of Vl and V2 practicallyinstantaneously assume the values indicated by the curves at thebeginning of the interval t3-t4, because of the cross connection of thecircuit. As shown in Fig. 4B, the grid of VI is'taken far below thecut-off and so the plate voltage remains constant at the maximum valuei-El. The condenser Ci charges up rather rapidly, and the grid potentialof Vl rises to the cut-off. At the same time, and in the same way thegrid potential of V2 fallsmuch more slowly due to the charging of C2 asindicated in Fig..4D.

The grid potential of Vl reaches the cut-off at time tl, the` platepotential having remained constant throughout the interval t3-t4 at themaximum value El. At the instant t4, anode currentbegins to now in VIandthe potentials all change practically instantaneously in the reversedirection. As the grid potentialof V2 has fallen during the intervalt3-t4, it will be suddenly reduced some way below the cut-off, and thecorresponding plate potential will assume the maximumvvalue +E1.Similarly the grid of VI will be taken suddenly up to a high positivepotential well above the saturation point of the valve, the platepotential of which will assume its minimuml Value -l-EB. Meanwhile, thegrid potential of VI has returned practically to the bias 'value -e ina. relatively short time duev to the grid current, and the correspondingplate potential has risen to the value E2. tential. of V2 will alsorise. but as the voltage difference EZ-E3 is small compared with thedrop of the grid voltage of V2 below the cut-off, the grid of V2 willremain below the cut-oil and willblock any further operation until theend of the relatively long period t4-t5 dened by the time constant C2,R2. At t5 the grid voltage of V2 reaches the cut-oil', when the cycle isrepeated. It will be noted that during the period t3-t4 the platevoltage of V2 assumes the minimum value +E3, since the grid voltage ristaken above the saturation point. It will remain substantially constantat this value until the grid potential has had time to fall below thesaturation point. It will thus be seen that nearlyv symmetrical squaretopped impulses are generated at the plate .of Vl and practicallysymmetrical inverted impulses are generated at the plate of V2. Thefrequency oi repetition is controlled by the timing circuit C2, R2 whichoperates by blocking the valve V2 for the desired period. It ,will beclear that the grid bias of this valve has to be adjusted appropriatelyso that the curve of Fig. 4D is suitablylocated with respect to thecut-off and saturation points of the valve.

It will also be clear that the periods t3-t4 and t4-t5 are dependentupon the properties of the valves as well as upon the timing circuitsCl, RI and C2, R2 and will accordingly be difficult to maintainconstant.

When the delay network output circuit is short circuited, the reflectedimpulses are of sign op posite to that ofthe original impulses, and whenit ls'left open they are of the same sign. The network LI will bearranged, for example, to pick up the impulses shown in Fig. 4C. Thede-Y lay will be so arranged that the leading edge of each reversedreilected impulse returns after the time corresponding to the desiredimpulse duration, and the timing circuit CI, RI (Fig. 3) will bearranged so that the duration of the impulses which would be produced inthe absence of the delay network is slightly greater than the desiredduration. Accordingly, the leading edge of the reversed impulse willreturn when the grid voltage shown in Fig. 4B has nearly but not quitereached the cut-ofi point, and will sharply bring it thereto thusunblocking the valve VI (Fig. 3), and completing the impulse after atime defined by the delay network and not by the timing circuit.Similarly, network L2 may be arranged to pick up the impulses showninFig. 4A and to return them (with the 'same sign) after a time equal tothe desired repetition period. 'Y

The timing circuit C2, R2 (Fig. 3) will be so arranged that the gridvoltage of valve V2 (Fig. 4D) in the absence of the network L2 wouldhave reached thecut-oii a little later than the return of the leadingedge oi? the reilectedv impulse, which on its ,arrival sharply unblocksthe valve V2 and causes the generation oi a new impulse after a timedefined by the network L2 and not by the timing; circuit C2, R2. 'I'hesetimes may be very precisely determined by suitable choice of the-networkconstants, and the variations due to external factors (e. g.temperature) can easily y be made negligible by appropriate design ofAthe network elements. The principal requirement for the timing circuitsis that the times defined by them shall notat anytime become less thanthev corresponding times deiined by the delay The grid po- 5 llowed tobecome greater, a limit must somewhere I y'be set to variation in thisdirection, otherwise -'the delay networks may cease to exert any con-t5:A trol. This limit will not, however, be so narrow as to be difcult.to meet, with the circuits in ordinary use. y A

Now referring more specically to Fig. 3, it will be noted that theresistances Rl and R2 have 10,--each been tapped.- so as to divide theminto two portions rl andRI-rl, r2 and R2--r2, respec-` jf tively. Thedelay networks LI and L2 are bridged across rl and 12 respectively. `Thearrangements for delivering the impulses to a load may be 15; made in anumber of well known ways such, for example by means oi an amplifyingvalve not "Tj vshown Whose grid is coupled to the plate of one of thevalves Vl or V2 through a condenser C3; or by direct connection to oneof the resistances P. The resistances rl and r2 should preferably bechosen so that each delay; network is terminated at its input terminalsz with an impedance equal jY-,lto its image impedance in order to avoidmultiple reilections. Y Assuming as before that the duration of theimpulses is defined .by the timing circuit Cl, Rl and the repetitionfrequency by C2, R2, then acncording to the previous explanation LIshould be short circuited, and L2 should be left uncon- ,nected at theoutput terminals 3. By suitably choosing the ratios Tl/Rl and 12/R2Vthestabilising impulses may be given suitable amplitudes. Alternatively theamplitude may be controlled by terminating the network at the output i:terminals 3 with a suitable impedance by which means the amplitude ofthe reected impulses can be adjusted.

Y Fig. 5 shows an arrangement alternative to Fig. 3 in which the timingcircuits are placed in the cathode circuits of the valves, andtheoperation will be similar. If the duration Lof the impulses is extremelyshort as compared with the period of repetition, it may not be possibleto get a sufcient drlop in the. grid potential of V2 in Fig. 3 in Y thetime :f3- t4 (see Fig. 4) to allow the circuit to remain .blocked forthe period lfd-t5. By connecting the timing circuit C2, R2 in serieswith the cathode a larger drop can be obtained in the time because theyplate current is then :available 5 0.for charging the condenser insteadof the grid current. With this arrangement the timingjcircuit CI, RIcould be left connected as shown in 3 but an additional advantage ofFig. 5 is .that the resistance RI and rR2 no longer shunt u-the platecircuit resistances P which in some cases may limit the possibilities ofthe circuit. y While, at the times t3 and t4, the drop and rise,respectively, in the cathode potential for lnv stance of valve Vl willbe gradual aridalong an 6g .1 exponential curve, it is possible toobtain a sharp impulse by inserting a small resistor rl, r2 in seriesWith the two timing circuits RI, CI and R2,

CA2. The voltage drop across this resistorwill'be y proportional to thatacross` the associated plate 65., resistor P and the potential of theungrounded terminal of rl, r2 will vary inversely with the respectiveplate voltage given in Figs. 4A and 4C. It is of cours-e necessaryto'make the ratio. between P and rl. or r2 suilciently'large so v.as not7gfappreciably to reduce the grid voltages of the two valves. l-

, .In the circuit arrangement of Fig. 5, a network L l'` has its inputterminalsl 2 connectedfacross the resistor rl and va. network L2 issimilarly .con- 7s; nectedacross the resistor r2.. Network LI.' servestcr. stabilize ther. duration.l ci the. impulses and: diitiers.: frontnetwork: Ll?, shown: in other: embodiments: of thee invention, inthatit's, output; tenninels-.3`l areopen circuited; accordingly; networkL2!` is provided for stabilizing. the; repetition frcquency:h ofithemultivibraltorrand isshortcircuitedl in contrast with network L2shown elsewhere in the drawings. Thee reason;` for' this will' be clearfronnthe foregoing; inviewci-thezfact that a posi;-V tivesynchronizingpulse in*l the* cathode ycircuit' is necessaryl for driving the-=associated1 gridy below cut-off'.

Experience shows that aV very good way of synchronislng' a.multivibrator is to apply' Snychronislng signals simultaneously toA the;plates of the: two valvesV-'IIl andrVZ; This may be done according to?the present invention, as shown inV 63 wherein the network Lek-suppliesthe synchronising': signals inl= the form' off the reflected impulses.Since theA` plate. resistances4 l?" of both valves are" equal; .the dropacross:v the resistor r3: in series therewith' will'vary asthedii'erencebetween the plate7 voltages off VI` and V2, correspending attime t3 to the value i will be without eiect due to the large positivevoltage simultaneously"appliedv thereto by wayl of condenser C2.

With the network L3 thus takingV the place of L2, L2' in Figs.,3 andA5f, aA second: network Ll orl LI may be connected in. the. same manneras in themodications lpreviouslydescribe'd, Thus, a networkl LI is shownin- Fig; 6 connected across the resistor' rl,.and its functionincontrolling the duration lof'feachA impulse is exactly as describedearlier in this specification. Alternatively, network L3 may beshort-circuited at terminal 3 and designedfor stabilizing the impulse.duration, and LIY will then be advantageouslyV replaced' by an opencircuitedl delay network L2 adapted'tdmain. tain a desiredl frequencyof'repetition.

Thus it will be` seen that by using :any one of the threedelayarrangements'shownin Figs'. 3; 5 and' 6' either the duration. orV theirequencyoi reflection ofr the impulses may be stabilised, or by usinganyY two of them, `both may belsimultaneously stabilised.

In Fig. 7' isV shown a different type of multivibrator circuit' to whicha` delay network is applied. This` circuit consists of a jthree stageampliercomprising; valves Vl', V2 and' V3 with a back' couplingA throughcondenserCl from thecathodeof V3 to that ci Vl; The valves'V24 and V3iare connected as cathode followers, and this arrangement isAparticularly suitable for the production of impulses with very high peakpower by the use ofv valves of low power rating. Valve V2- mayv beapentode havingy a normal voltage on the screen gridy but with anabnormally high plate voltage, but inA Fig. '7 this valve' is sho-wn forsimplicity as atriod'e for the purpose of' ex plaining the circuit,whose working as regards the invention is not conned to the'r use of anyparticular valves or method' of operating them. The cathode followervalves V2 and V3 may have the samepl'atevoltage as V11. and are providede; witlr cathode circuit` rv 'stances RC2 and. RCBi so4 chosen thattheipotenti'ali. of the: cathode is always verynearly equal to thepotential ofthe corresponding grid.4 By making the plate resistance PofVIVv large enough, the. desired voltage amplitude of the impulses isobtained, and the power is, subsequently amplified in V2 and1V3` withoutmuch changing'the voltage; Itis possible" togive RC2 @a value at leastten tiinesflower than?, and RC3 a value at least 501 timesA lower. Itwill? thus be seerrthat since thev voltageno the impulses is notmaterially changed, the power may bemultiplied' atleast 50 times.

Thev amplifier may be' converted into a multi.- vibrator in Fig. '7 byconnecting a condenser 'C.|1 from a suitable tap on' the cathoderesistanceI RC3 [of V3 to the cathode of VI. which is connectedtofground'through the resistance R'I and r3. The valves V2' and V3 arecoupled-through condenser; C2 land resistance R2 as shown; The timing'vcircuits are CI, RI and C2, R2' as before; and C2, R21' could have beenplaced elsewhere, for inst'ance'between valves Vl and V21. The circuit'operates inv substantially the same manner as those previouslyvdescribed, namely, an operating parameter such asl-the repetitionIfrequencyl is determinedby the yapplication of a synchronizingV impulseto the input circuit foivalve Vl. The timing circuits Ci, Rl and C2, R2could be in terchanged' if desired;

stabilisation is effected in' the mannerY explainedj with reference'`t'o Figs. 1aorv 2b. An unbalanced delay vnetwork L is connectedv with''its input terminal 2 in series with the 'cathode resi-stance RC3and'its-out'put terminal 3 in series with the. resistance Rif. Thecommon terminal 5T is connected to ground'. The delay will beso chosenthat th'ev'alve Vl is triggered atA ther right time iorstaloilising therepetition frequency, the delayed impulses beingl of' the same' sign asthe original ones. The networkl will be' preferably connectedateitherend toy resistances r2` andir34 as shown having values such thatit will be termi"- nated with its image impedance in order to avoid'reflections'.

In Figure 8 the circuit of Figure 7 has been modie'd somewhat in orderalso to stabilise'theduration of the impulses, .the cathode resistanceR`I is in this case connected to a tapping point 4. in L dividing itinto two portionsLl' and L2', asshown instead of to the output terminal'3. In this ease the input of network portion LI is connectedf across lapart ofthe cathoderes'istance ofVfl'r'epresented by resistor r3, just asthe input of portion L2' is connected across a part of the cathoderesistance of' V3 consisting of the resistor r2'. The impulses appliedto terminal 4' will be reflected at the end 3 and returned to thecathode circuit of Vl where they may be utilized for controlling theoperation of the multivibrator circuit. In order to reverse thepol'arityofthe reflected impulses, the network portion Ll may bevshort-circuited,e. g. by connecting terminal3 directly to gro-undl as shown, It may,however in some cases be groundedl through an irnlpedance of a suitablevalue 'inserted between terminals 3f and 5. The delay in the portion L2is chosen to stabilise the repetition frequency, and thev delay in theporti-on LI for theY signals reii'ecte-d atA the short circuited endback to thetapping 4i is chosen to stabilise the duration of`theimpulses.

In the embodimentsv of the invention whichy havek been so far described,the impulses, reflected or transmitted; which constituteA the syn-`escasos chronising signals will often be of substantially the same formas the generated impulse applied tothe delay network. This however isnot essential and may not always be desirable. By use of suitablydesigned networks or arrangements, stabilising signals of modified formmay be produced. For example it may be convenient touse a signalobtained by derivation of the generated impulse (that is, thestabilising signal has a form defined by a curve which is the firstderivative of the curve defining the original impulse). Such signals maybe produced by well known means: one way of doing this is kto use delaynetwork terminated in a condenser, in which case the reected impulse has(as may be demonstrated experimentally) a form which is roughly thatofthe derivative of the original impulse. In Fig. 9 is shown anothermultivibrator, primarily intended as a harmonic generator and having atuned output circuit. This may be staprinciples explained above, but anlalternative method may also be used as shown in Fig. 9. The circuit isshown with two valves VI and V2 but is otherwise similar to Fig. 7 andthe corresponding elements are similarly designated. The plate currentof V2 is taken through the primary winding of a transformer T tuned to asuitable harmonic by means of condensers C3 and Cfi. The secondarywinding is connected back to the grid of valve Vl. The alternatingpotential of this harmonic may be caused' to unblock the circuit at theright time by suitable arrangements in a manner similar to thefunctioning of the impulses delayed in the network as previouslydescribed.

The ampliers shown -in Figs. "l, 8 and 9 may take various forms withoutlaffecting them-ain principles of the invention. i 'Ihus either mayconsist of any number of stages of amplication, and any of such stagesmay be arranged as Acathode followers. Further the-back coupling may bemadel between anytwo-stages not necessarily the rst and the last, andthe timing circuits can be located in any of la number of differentplaces bilised using delay networks according to` the in the circuit. Itis alsoy not essential Athatthe delay networks or other stabilisingmeans should be connected between the same stages as the back coupling.

It will be understood that the circuits described above have been givenfor illustrating the application of the invention, and are not intendedto limit to the particular arrangements shown. Many other circuits andapplications will occur to those skilled in the art.

What is claimed is:

1. A thermionic multivibrator for generating regularly repeatedrectangular electrical impulses of short duration comprising twothermionic valves each having an anode, a cathode and at least onecontrol electrode, the control grid of each valve being connected to theanode of the other valve with a timing condenser in said connection, atiming resistance connected between each control grid and the ground, acommon source of anode potential for said valves, means for applying abiasing potential to said control grids, individual resistances beinginterposed between said common source and each anode, and a delaynetwork for stabilizing the operation of said multivibrator and operatedby impulses generated by said multivibrator without any Iother source ofelectric power, said network being connected to receive impulsesgenerated by one of 4'said valves and. after.A vto deliver said impulsesto one of said valves.

a predetermined interval,

2. A thermi-onie multivibrator las claimed in claim 1 wherein said delaynetwork lis connected with its input circuit in series with part of thecircuit of one of said valves, the other end of v,the network being leftunconnected whereby impulses are reiiected back through the network tostabilise the frequency of repetition of said impulses.

3. A thermionicmultivibrator as claimed in 'claim 1 whereinsaid delayVnetwork is connected 4with its input circuit in series with part of thecircuit of one of said valves, the .other end of the .network beingshort circuited whereby impulses are reflected back through said networkVto stabilise the duration of said impulses.

4,A thermionic multivibrator as claimed in claim 1 wherein said delaynetwork is connected -with its input circuit in Iseries with said commonsource of anode potential.

5. A thermionic 'multivibrator as claimed in .claim 1 wherein said delaynetwork is connected with its input circuit in vseries with the controlelectrode circuit of one of said valves.

6..A thermionic multivibrator as claimed in claim. l wherein said delaynetwork is connected with its input circuit in series with said commonsource of anode potential, and having a second delay network connectedwith its input circuit in yseries with the control electrode circuit ofone of said valves, one network having its other end left. unconnectedfor stabilising the frequency of repetition of said impulses and theother network having itsother end short circuited fior stabilising,valve Yofzthe amplier having a resistance in its cathode circuit, meansfor supplyingspace current to each 0f said valves, means for applyingagrid biasing potential to each of said valves, means including a timingcondenser connecting the cathode of the rst valve with a tapping pointon said resistance, means including 1a timing resistance connecting thecathode of said rst valve to ground, a timing circuit coupling thecontrol electrode of a subsequent valve with the immediately precedingvalve in the amplifier, said coupling circuit comprising a seriescondenser and a shunt resistance, and a, delay network for stabilisingthe operation of said multivibrator and operated by impulses generatedby said multivibrator without any other source of electric power, saidnetwork being connected with its input receiving rimpulses generated byone of said valves and its youtput delivering impulses to another ofsaid valves after a predetermined interval.

8. A thermionic multivibrator as claimed in claim 7, having anunbalanced delay network with a plurality of terminals including a firstterminal of fixed potential and a second and a third terminal both ofvariable potential, said rst terminal being connected to ground, saidsecond terminal being connected to said tapped resistance.

9. A thermionic multivibrator as claimed in claim 7, having anunbalanced delay network with a plurality of terminals including a rstterminal of iixed potential and a second and a third terminal both ofvariable potential, said first terminal being connected 'to ground, saidsecond terminal being connected to said tapped resistance and said thirdterminal being connected to the timing resistance associated with lthecathode of the first valve.

10. A thermionic multivibrator as claimed in claim 7, having anunbalanced delay network with a plurality of terminals including a iirstterminal of fixed potential, a second terminal at one end of saidnetwork, a third terminal at an intermediate point on said network, saidsecond and third terminals being both of variable potential, and afourth terminal at the :other end of said network, said rst terminalbeing connected to ground, said second terminal being connected to saidtapped resistance, said fourth terminal being connected to ground, :andmeans connecting said third terminal with the timing resistance of saidfirst valve.

11. A thermionic multivibrator of the self-oscillatory type adapted toserve as a primary generator of electrical oscillations, for generatingregularly repeated rectangular impulses defined by a plurality ofparameters including impulse duration and repetition frequency,comprising a rst valve having a cathode, an anode and at least onecontrol grid, a second valve having a cathode, an anode and at least onecontrol grid,

means including a timing circuit in the gridcathode circuit of the firstvalve for coupling the input thereof to the output of the second valve,means including a timing circuit in the gridcathode circuit of thesecond valve for coupling the input thereof to the output Iof the firstvalve, the said valves being responsive to synchronizing sign-als forstabilizing at least one of said parameters at a desired value, andmeansV for producing the synchronizing signals, said means including adelay network consisting of a combination of linear bilateral impedanceelements for deriving an impulse from the output of one of said valvesand applying said impulse after. a predetermined time interval to aninput electrode of one of said valves.

12. A thermionic multivibrator of the selfoscillator;7 type adapted toserve as a lprimary 12 generator of electrical oscillations, forgenerating regularly repeated rectangular impulses delined by aplurality of parameters including impulse duration and repetitionfrequency, comprising la first valve having a cathode, an'anode and atleast one control grid, a lsecond valve having =a cathode, an anode andat least one control grid,` means including la timing circuit in thegrid-cathode circuit of the first valve for coupling the input thereofto the output lof the second valve, means including a timing circuit inthe grid-cathode circuit of the second valve for coupling the inputthereof to the output of the first valve, the said valves beingresponsive to synchronizing signals for stabilizing lat least one ofsaid parameters at a desired value, and means including a resistanceelement in series with an electrode of one of said valves for producingthe synchronizing signals, said last means further including at leastpart of a delay network con'- sisting of a combination of bilateralimpedance elements adapted to derive an impulse from said resistanceelement and to return it thereto after a predetermined time interval andwith` a predetermined polarity.

BENT BLOW JACoBsEN.

MAURICE MosE LvY.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,158,285 Koch May 16,19392,212,173 Wheeler Aug. 20, 1940 2,212,420 Harriett Aug. 20, 19402,212,967 White Aug. 27, 1940 2,221,666 Nelson Nov. 12, 1940 2,265,996Blumlein Dec. 16, 1941 1,744,335 om Jan. 2a, 1930 2,283,653 Siemens May19, 1942 2,265,290 Knick Dec. 9, 1941 2,401,405 Bedford June 4, 19462,404,047 Flory et al. July 16, 1946

